As is well known, government regulations require vehicles equipped with internal combustion engines to have emission monitoring systems conventionally known as OBD (On-board Diagnostic Systems) to advise the operator of the vehicle when the gaseous pollutants or emissions produced by such vehicles exceed government regulatory standards. Government regulatory standards set emission threshold levels which the vehicle cannot exceed when operated pursuant to a specific driving cycle such as that set forth in a FTP (Federal Test Procedure). The FTP requires that the vehicle be operated at various acceleration/deceleration modes as well as at steady state conditions.
One of the principal components of the vehicle's emission system is the catalytic converter, typically a TWC (Three Way Catalyst--NO.sub.X, HC, CO). TWCs store oxygen when the engine operates lean and release stored oxygen when the engine operates rich to combust gaseous pollutants such as hydrocarbons or carbon monoxide. As the catalyst ages, its ability to store oxygen diminishes and thus the efficiency of the catalytic converter decreases. Currently, the diagnostic criterion mandated by legislation for determining the state of the catalytic converter is the efficiency at which the catalytic converter destroys hydrocarbons in exhaust gas. Catalytic converter failure is defined under current regulations as occurring when the hydrocarbon emissions level exceeds 175% of the maximum emissions level permitted for that vehicle when it was new.
There is currently no low-cost, reliable sensor for determining the hydrocarbon concentrations in the automotive exhaust gas streams. Conventional monitoring systems typically used by the automotive vehicle manufacturers today include the placement of an EGO (Exhaust Gas Oxygen) sensor either within or downstream of the TWC to sense the oxygen content of the exhaust gas after it leaves the catalytic converter. The signals generated by the downstream EGO are compared with the signals generated from an EGO positioned well upstream of the TWC and typically used to sense oxygen content readings in the exhaust gas for adjusting the air/fuel ratio content of the vehicle's engine. The upstream and downstream EGO signals are adjusted for the time it takes the exhaust gas to travel from the upstream EGO to the downstream EGO and the adjusted EGO signals are then compared to ascertain the storage capacity of the TWC when the engine is in either a lean or stoichiometric operating mode. It is now generally acknowledged that such systems provide "rough" or "crude" estimates of catalytic converter efficiencies which, in turn, have to be correlated to the regulated hydrocarbon emissions. Importantly, it is generally considered that EGO based monitoring systems will become increasingly unsatisfactory as legislation (state, federal and European) decreases the allowable vehicle emissions such as those now required or contemplated under LEV (Low Emission Values) and ULEV (Ultra Low Emission Values) requirements.
Alternative systems now under development to meet LEV and ULEV requirements are considering the use of "calorimetric" or "pellistor" type hydrocarbon sensors in place of EGO sensors. The present invention relates to such systems.
Reference can be had to U.S. Pat. Nos. 4,036,592; 4,416,911; and 4,329,874, incorporated by reference herein, for description of the principles used in calorimetric sensors to determine the presence of certain gaseous compounds in a gas mixture, calorific content of fuel gas, etc. Briefly, calorimetric or pellistor type sensors are combustible gas sensing devices which operate by catalyzing an exterior surface of a temperature measuring element such as a thermocouple or a resistance thermometer. The combustible gas, at reaction temperature, diffuses to the catalyst where it is oxidized and in the course of oxidation, liberates heat, i.e., principally an exothermic reaction. The temperature sensing element detects the resultant temperature rise and provides a varying signal which is proportional to the temperature rise and in turn correlated to the concentration of the combustible gas in the gas mixture.
It is common in the design of todays' pellistor sensors to use electrical heaters so that the pellistor can operate above ambient temperature. It is known to employ a pair of sensing elements, one catalyzed to generate a sensing signal and the other not catalyzed to generate a reference signal. The difference in signals between the two sensing elements is then a measure of the combustible gas concentration. This measuring technique is discussed and validated by Chen et al., Sensors and Actuators, (1989) p. 237-248 incorporated by reference herein. It is also known to use an electrochemical cell to function as an oxygen source to insure the presence of sufficient oxygen to generate combustible mixtures when the engine is operating rich. Reference can be had to U.S. Pat. Nos. 5,476,001 and 5,505,837, incorporated by reference herein, for exemplary descriptions of such sensors.
Reference should also be had to U.S. Pat. Nos. 5,444,974 and 5,265,417 which describe specially developed calorimetric or pellistor type sensors used as hydrocarbon sensors in emission systems which monitor catalytic converter efficiencies.
Now, it is to be appreciated that within the vehicle's exhaust stream, the concentrations of carbon monoxide and hydrogen (combined) are about ten times as great as the hydrocarbon concentration. Typical gas concentrations for an internal combustion engine operating under closed loop control are about 750-1000 ppm (on a Cl basis) hydrocarbons and about 0.8% carbon monoxide. With the LEV and ULEV requirement stemming from OBD-II regulations requiring detection of small amounts of hydrocarbons (on a ppm basis) it becomes increasingly difficult to ascertain which combustible in the exhaust stream sensed by a conventional pellistor constitutes the regulated hydrocarbon emission. In this connection, attempts to statistically extrapolate the hydrocarbon concentration from the total combustibles sensed by the pellistor are simply that, an extrapolation which is inherently flawed due to inabilities to control variable parameters such as temperature ranges of the exhaust gas. Similarly, attempts to control the temperature of the exhaust gas so that temperature differentiations can be utilized to sense only specific combustible type reactions, while acceptable in theory, are simply not practical to implement in an automotive vehicle. For example, while heaters are conventionally provided to maintain a lower reaction temperature, there is no control of the upper temperature range.
Accordingly, a co-pending patent application of the inventor provides a solution to this problem by enabling a pellistor sensor to determine hydrocarbon concentrations in the presence of large quantities of carbon monoxide. This is accomplished by using a pair of temperature sensing elements within the sensor with each sensing element bearing a different oxidation catalyst. One temperature sensing element is coated with a catalyst having a high activity for oxidation of all the combustible gases present in the gaseous exhaust stream, i.e., CO, hydrogen, hydrocarbons. The second temperature sensing element in the sensor is coated with a catalyst which is efficient for oxidation of carbon monoxide and hydrogen but is not active for oxidation of hydrocarbons. By subtracting the differences in the temperatures recorded between the first and second temperature sensing elements, a temperature differential can be quantified and that differential is proportional to the concentration of the hydrocarbon gases within the exhaust gas. The concept is technically sound and provides, in theory, an efficient method and apparatus for accurately determining the hydrocarbon concentration in an exhaust gas stream.
In practice, reliable correlation to hydrocarbon concentrations have not occurred principally because of thermal mismatches between the first and second temperature sensing elements. That is, the thermal readings for each catalyzed sensing element do not retain their predicted proportionality to one another. They are thermally "mismatched". A number of reasons can be attributed to the thermal mismatch including but not necessarily limited to differences in the size of the screen printed catalyst areas, misalignment of the catalyst coated areas, differences in thermal coupling within the sensor itself, etc. In theory, it is possible to correct for the thermal mismatch on a sensor by sensor basis through calibration using various gases of known combustible make-up. This is not commercially viable. More significantly, even if calibrated into a thermal match, drift over time will not be uniform for both catalysts and thermal mismatch will again occur.